Method for shifting the instant of commutation for a sensorless and brushless direct-current motor as well as a system for implementing the method

ABSTRACT

The present invention provides a method for shifting the instant of commutation for a sensorless and brushless direct-current motor ( 1 ), whose stator windings are fed by a multi-phase converter connection. The converter connection includes an output stage control ( 2 ), a commutation logic ( 3 ), a phase selector ( 4 ), and phase discriminator ( 5 ). A commutation detection ( 6 ) is supplied at one input ( 46 ) with the instantaneous value of the voltage induced in a phase, the instantaneous value being determined by the phase selector, and at a second input ( 47 ), with a reference voltage (U ref ) for comparison. The reference voltage (U ref ) can be changed by a commutation shift ( 7 ) in correspondence with a specific characteristic curve ( 71 ). A manipulated variable (U st ) is supplied by a manipulated-variable calculation ( 8 ) to the commutation shift ( 7 ) as a function of the setpoint speed (N setpoint ) of the motor. The commutation shift takes place in an advantageous manner in a parabola shape. As a result of the setpoint value-dependent commutation shift, a high torque is provided also in the case of high rotational speeds and a heavy load, and the torque ripple is kept to a minimum.

BACKGROUND INFORMATION

The present invention starts out from a method for shifting the instantof commutation for a sensorless and brushless direct-current motor whosestator windings are fed by a multi-phase converter connection accordingto the definition of the species in claim 1 and also relates to a systemfor implementing this method according to claim 6.

DE 39 40 568.9 A1 describes a circuit configuration for operating amulti-phase synchronous motor at a direct-current supply. In thiscontext, the phases are successively connected to the direct voltage andcommutating circuits corresponding to the rotor position are controlledin such a manner that they overlap with respect to time for commutatingsubsequent phases, and at least one of the commutating circuits in thecommutation range is clocked in such a manner that the average value ofthe current increases in the forward commutating phase and decreases inthe reverse commutating phase. As a result of this overlapping andclocking of the switching signals in the commutation edges, there isless switching loss and a reduction in noise.

For sensorless and brushless direct-current motors, the instant ofcommutation is typically determined by measuring the induced voltage ina particular non-current carrying stator winding phase. In this context,this induced voltage is compared to a reference voltage that is derivedfrom the actual value of the rotational speed. In this connection,significant power notches and ripples in the torque can occur,particularly in the case of large loads and high motor speeds. This isextremely disadvantageous.

The object of the present invention is to provide a method that enablesthe instant of commutation to be shifted for a sensorless and brushlessdirect-current motor so as to prevent or significantly reduce the powernotch, and decrease the torque ripple.

SUMMARY OF THE INVENTION

With respect to the related art, the method according to the presentinvention for shifting the instant of commutation for a sensorless andbrushless direct-current motor having the characteristic features ofclaim I has the advantage of an increase in power with a constantmagnetic circuit and identical motor mechanics, and of a reduction inthe torque ripple by adapting the commutation threshold to an optimumcurrent waveform. Advantageously, there is also no power notch observed,as is in the case of a commutation shift dependent on the measured motorspeed.

In the method according to the present invention, this is principallyachieved in that commutation is detected by comparing the voltageinduced in a non-energized stator winding phase to a reference voltage,and in that the reference voltage is changed in dependence upon thesetpoint value of the motor speed and/or the manipulated variablecalculated therefrom.

Advantageous further refinements and improvements of the method statedin claim 1 are rendered possible by the measures specified in theadditional method claims.

According to a particularly advantageous and preferred specificembodiment of the method according to the present invention, the instantof commutation is shifted ahead with respect to time in such a mannerthat an optimum current waveform is achieved, i.e., optimum particularlywith regard to an increase in power and/or a reduction in the torqueripple.

According to a particularly effective and advantageous embodiment andfurther refinement of the method according to the present invention, theinstant of commutation is shifted in such a manner that the referencevoltage is raised in the shape of a parabola.

In a further advantageous embodiment of this method feature, given apulse width modulation of the current supplied to the stator windings,the reference voltage is raised in the shape of a parabola, beginning ata pulse width modulation ratio of about 90 to 95%, in particular 93%.Raising the commutation threshold in the shape of a parabola has theadvantage that it results in a smoother transition to thepre-commutation state.

According to a further advantageous feature of an exemplary embodimentof the method according to the present invention, besides being used forchanging the reference value for the instant of commutation, themanipulated variable determined in dependence upon the setpoint value ofthe rotational speed is also used for adapting the current supply to theindividual stator winding phases, raising it or lowering it accordingly.

A preferred system for implementing the above-explained method with itsdifferent modifications includes a sensorless and brushlessdirect-current motor that is fed by a multi-stage converter connection,which, for its part, includes an output stage control, a commutationlogic, a phase selector, and a phase discriminator, and is characterizedin that a commutation detection is provided which is supplied at oneinput by the phase selector with the instantaneous value of the voltageinduced in a non-energized phase and, at a second input, with areference voltage, for comparison, and in that the reference voltage canbe changed by a commutation shift in accordance with a specific curve, amanipulated variable being supplied to the commutation shift by amanipulated-variable calculation as a function of the setpoint speed ofthe motor.

In an advantageous embodiment of this system according to the presentinvention, it is provided that in the commutation shift, the referencevoltage changes in accordance with a parabola, in particular, it isincreased.

Given a pulse width modulation of the current supply to the individualstator winding phases of the motor, one advantageous embodiment of thissystem configuration provides that the reference voltage is increased inthe shape of a parabola, starting from a pulse width modulation ratio ofabout 90 to 95%, preferably 93%. These percent values apply for aspecific magnetic circuit design. Other designs of the magnetic circuitcan result in significantly different values.

In a further advantageous and particularly effective configuration ofthe system according to the present invention, calculating themanipulated variable yields, as a non-linear function of the setpointspeed of the motor, a manipulated variable that, on the one hand, issupplied to the commutation shift as an input, and, one the other hand,is supplied to the commutation logic for adapting the current supply tothe stator winding phases of the motor.

BRIEF DESCRIPTION OF THE DRAWING

The method according to the present invention and the system forimplementing this method are more closely explained in the followingdescription using an exemplary embodiment represented in the drawing.The figures show:

FIG. 1 shows a schematic block diagram for the commutation shiftaccording to the present invention;

FIG. 2 shows a diagram having the reference voltage as a function of themanipulated variable and/or the current in parabolic dependency; and

FIG. 3 shows different diagrams of the voltage curve of the inducedvoltage in one phase and of the current energization of this phase, andthree different current waveforms for different reference voltages anddifferent motor speeds, in general, current waveforms for differentcommutation thresholds.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

In a block diagram, FIG. 1 represents the commutation shift according tothe present invention. A direct-current motor 1, which is sensorless andbrushless, is fed by a multi-stage converter connection. For its part,this multi-stage converter connection, which can be three-phased andsix-pulsed, for example, includes an output stage control 2, acommutation logic 3, a phase selector 4, a phase discriminator 5, aswell as a commutation detection 6 as the main components. A MOSFETtransistor 22 is symbolically represented in output stage control 2. Theoutput stage control supplies motor 1 with energy via a multiple line21. Branching off of these lines, the respective instantaneous value ofthe voltage induced in a non-current-carrying phase is supplied via onemultiple line 23, one of the, for example, six phases being selected ineach case for this purpose by phase selector 4. This instantaneous valueof the respective phase is transmitted by phase selector 4 via line 46to a first input of commutation detection 6. A reference voltageU_(ref), which is formed from adding the battery voltage supplied tomotor 1 to the voltage from the commutation shift, is fed via line 47 toa second input of the commutation detection.

For its part, commutation shift 7 is supplied by a manipulated-variablecalculation 8 via a line 87 with a manipulated variable U_(st). Setpointvalue N_(setpoint), of the rotational speed of motor 1 is available asan input value via line 80 for manipulated-variable calculation 8.Manipulated variable U_(st) is determined in accordance with non-linearcurve 81, which is represented in manipulated-variable calculation block8, from this setpoint variable N_(setpoint). This characteristic curve81 is manipulated variable U_(st), which is to be plotted over speedsetpoint N_(setpoint). Entered in block 7 of the commutation shift is acharacteristic image in which manipulated variable U_(st) obtained inblock 8 is plotted on the horizontal axis, and reference voltage U_(ref)is plotted on the vertical axis as a function thereof The plottedcharacteristic curve 71 is preferably parabolic. The output value ofmanipulated-variable calculation 8 is fed via a line 83 to commutationlogic 3 to increase the currents for output stage control 2 and thepower transistors 22 contained therein, in correspondence with thepredefined rotational speed N_(setpoint) in the commutation logic, thecurrents then being supplied with the correct timing via lines 21 tomotor 1.

The clock-pulse generation for commutation logic 3 is carried out byphase discriminator 5, which, as an input on line 65, generates theresult of the comparison of reference voltage U_(ref) on line 47 and theinstantaneous, phase-induced phase voltage on line 46 by comparator 61in commutation detection 6. The output signal of phase discriminator 5,which is characterized by six different phases of pulse generationwithin block 5, is supplied via line 52 to commutation logic 3 and vialine 54 to phase selector 4. As a result, phase selector 4 is adjustedto the correct phase for the commutation detection.

FIG. 2 provides another larger and more exact view of the characteristiccurve represented in FIG. 1 within commutation shift 7, in a slightlydifferent shape. In this context, the percent value of the pulse widthmodulation is plotted on the horizontal axis, this corresponding tomanipulated variable U_(st) for characteristic curve 71 withincommutation shift 7 in FIG. 1. Plotted on the vertical axis is referencevoltage U_(ref), which is supplied to commutation detection 6 via line47. The parabolic characteristic curve of reference voltage U_(ref)begins to climb, namely in the shape of a parabola, starting at a pulsewidth modulation ratio of about 90%, especially 93%. This parabolicraising of the commutation threshold, i.e., of the reference voltageU_(ref) supplied via line 47, has the advantage that the transition tothe pre-commutation state is smooth. Pre-commutation state means thatthe instant of commutation is advanced from its usual temporal state toan early start.

FIG. 3 shows different diagrams and different current waveforms fordifferent commutation thresholds. In the top diagram designated as A,voltage U_(ind), which is induced in a phase U, for example, is plottedover the electrical angle. U_(bat) designates the battery voltage of amotor vehicle or the normal voltage of a direct-current vehicleelectrical distribution system of a motor vehicle. U_(ref1), which isbelow the battery voltage, designates a first reference voltage, andU_(ref2) a designates a second reference voltage that is significantlyabove the value of battery voltage U_(bat). One can assume that voltagereference value U_(ref1) approximately corresponds to the value 0.00 inFIG. 2, and voltage reference value U_(ref2) in FIG. 3, diagram Aapproximately corresponds to the value 1.00 of reference value U_(ref)in FIG. 2.

In diagram B of FIG. 3, IU designates the on-time of the current forphase U. If commutation is performed to reference value U_(ref1), thecurrent and current waveform are plotted over time t in diagram C ofFIG. 3, the time corresponding, for this commutation threshold, to asetpoint speed of 1500 revolutions per minute, for example. Thus,current I₁₁₅ approximates the current waveform that, together with thevoltage induced in the same phase, causes the torque.

Plotted over time t in diagram D of FIG. 3 is current waveform I₁₃₀,which ensues at a commutation threshold of voltage U_(ref1) in diagram Aof FIG. 3 and at a rotational speed of 3000 revolutions per minute. Itis recognizable that the current can only slowly and weakly increase inthis phase as a result of the winding inductance at hand.

Plotted over time t in diagram E of FIG. 3 are the current and currentwaveform I₂₃₀, which ensues at a commutation threshold of U_(ref2) andat a rotational speed of 3000 revolutions per minute. It can be seenfrom the diagram that at the instant corresponding to voltage U_(refl)in diagram A, current I₂₃₀ is already built up to its full value, and,therefore, when the induced voltage increases, the full torque can bemade immediately available for use.

In accordance with the present invention, the instant of commutation canchange between those values that are between the points corresponding tocommutation threshold U_(ref1) and U_(ref2) in diagram A. As a result ofthe temporal shift forward, commutation is correspondingly prematurelyended by the current being correspondingly switched off, as can beclearly seen from diagram E of FIG. 3.

The function of the method according to the present invention and of thesystem according to the present invention is explained using the exampleof a air-conditioner fan motor for use in a motor vehicle. This isrepresented in detail in FIG. 1-3 and was already extensively describedabove. In this context, it is important that in the case of such ablower, the load and, as such, the phase current increase quadraticallywith the rotational speed. Thus, for example, current I₁₁₅ in diagram Cat 1500 revolutions/minute has a value of 3 amperes. Given a commutationto threshold value U_(ref1), this current at 3000 revolution/minute hasa value of 18 amperes according to diagram D in FIG. 3. It isrecognizable from this representation that the shift of the commutationthreshold is necessary to attain an optimum current waveform for alloperating states, and, as such, a high torque in the case of a smallertorque ripple. Such an optimum current waveform is represented bycurrent I₂₃₀ in diagram E of FIG. 3. As a result of such an optimumcurrent waveform, the ohmic losses and the switching losses in thesemiconductor circuit are kept as minimal as possible.

Due to the winding inductance, the phase current can only increase in alimited time. In the case of low rotational speeds, as is shown indiagram C of FIG. 3 by current I₁₁₅, this effect is not significantlynoticeable with respect to the function angle. First in the case ofaverage to high rotational speeds of about 1500 to 3000revolutions/minute, does the restricted current increase have a negativeeffect on the torque formation, the torque being M=cΦ*I, because at theinstant the full, induced voltage is reached, U_(ind)≈c*Φ, the phasecurrent is not yet built up. In diagram D of FIG. 3, this isparticularly represented by and readily recognizable from current I₁₃₀.Shifting the commutation threshold having a reference voltage U_(ref)that is greater than operating voltage U_(bat), as shown in diagram A ofFIG. 3, results in the phase current having already adjusted itself toits maximum value upon reaching the full, induced voltage U_(ind). As aresult, the maximum torque can also be attained. The continuous increasein the commutation threshold having value U_(ref) starting from adefined setpoint speed enables the torque to be increased with constantmotor mechanics.

Given drive systems having sensors attached, such an increase in torqueis not possible due to the fixedly predefined position of the sensor.For sensorless drive systems that function using commutation shiftingdependent on the actual value of the rotational speed, the disadvantageis that the commutation threshold is lowered in the case of a break ofthe speed due to a load increase. The resulting cancellation of thepre-commutation causes additional breaks in the rotational speed. Thisworsens the matter.

In contrast, the present invention in which the commutation shift iscoupled to the speed setpoint ensures that this effect does not occur.The method according to the present invention and the system for itsimplementation according to the present invention advantageously providea power increase with a constant magnetic circuit and identical motormechanics, decrease the torque ripple, prevent power notches, and ensurea smooth transition to the pre-commutation state by increasing thecommutation threshold in the shape of a parabola This is thenparticularly advantageous when the motor is used for application as afan in motor vehicles, where the fan load increases quadratically withthe rotational speed. In this case, the parabolic commutation shift isparticularly advantageous for the smooth load transition.

What is claimed is:
 1. A method for shifting an instant of commutation for a sensorless and brushless direct-current motor including stator windings fed by a multi-phase converter connection, comprising the steps of: detecting the instant of commutation by comparing a voltage induced in a stator winding phase in which no current is applied to a reference voltage; changing the reference voltage in dependence upon at least one of a setpoint value of a rotational speed of the direct-current motor and a manipulated variable calculated from the setpoint value; and shifting the instant of commutation such that the reference voltage is raised in a shape of a parabola.
 2. The method according to claim 1, wherein: with respect to a pulse width modulation of a current supplied to the stator windings, the parabola-shaped raising of the reference voltage begins at a pulse width modulation ratio of about 90 to 95%.
 3. The method according to claim 2, wherein: the pulse width modulation ratio is 93%.
 4. The method according to claim 1, further comprising the step of: adapting a current supply to individual stator winding phases in accordance with the manipulated variable in order to one of raise and lower the current supply accordingly.
 5. A system for shifting an instant of commutation, comprising: a multi-stage converter connection, including: an output stage control, a commutation logic, a phase selector, and a phase discriminator; a sensorless and brushless direct-current motor fed by the multi-stage converter connection; a commutation detection element, including: a first input supplied by the phase selector with an instantaneous value of a voltage induced in a non-energized phase, and a second input supplied with a reference voltage for comparison; a commutation shift element for changing the reference voltage in accordance with a specific curve, wherein in the commutation shift element, the reference voltage is changed in accordance with a parabola; and a manipulated-variable calculation element for supplying a manipulated variable to the commutation shift element as a function of a setpoint speed of the direct-current motor.
 6. The system according to claim 5, wherein: the reference voltage is increased.
 7. The system according to claim 5, wherein: with respect to a pulse width modulation of a current supply to individual stator winding phases of the direct-current motor, the reference voltage is increased in a parabola shape, starting from a pulse width modulation ratio of about 90 to 95%.
 8. The system according to claim 7, wherein: the pulse width modulation ratio is 93%.
 9. A system for shifting an instant of commutation, comprising: a multi-stage converter connection, including: an output stage control, a commutation logic, a phase selector, and a phase discriminator; a sensorless and brushless direct-current motor fed by the multi-stage converter connection; a commutation detection element, including: a first input supplied by the phase selector with an instantaneous value of a voltage induced in a non-energized phase, and a second input supplied with a reference voltage for comparison; a commutation shift element for changing the reference voltage in accordance with a specific curve; and a manipulated-variable calculation element for supplying a manipulated variable to the commutation shift element as a function of a setpoint speed of the direct-current motor, wherein the manipulated-variable calculation element computes the manipulated variable as a non-linear function of the setpoint speed of the direct-current motor, and the manipulated variable is fed, on the one hand, as an input to the commutation shift element, and, on the other hand, to the commutation logic to adapt a current supply to stator winding phases of the direct-current motor. 